Nowadays, the fields of technology and medicine can no longer be imagined without membranes. Frequently, separation processes can be carried out more advantageously by using membranes instead of other techniques.
Membranes of the most diverse materials and processes for their manufacture are known. Regenerated cellulose, polyamides, and polyolefins are among the materials from which membranes have been made. No matter how excellent membranes of these polymers may be for particular applications, for example, for dialysis of the blood of kidney patients, in many applications they have disadvantages that limit, or even rule out, their use. Such disadvantages of membranes from the aforementioned polymers may include lack of resistance to certain organic solvents or aggressive media such as, for example, strong acids or bases, as well as instability at high temperatures.
Because of the disadvantages of membranes made of organic polymers, membranes from inorganic materials have already been made and used. For example, silica membranes are known from West German Laid-open application (DE-OS) 2,462,567. Porous membranes of alumina are also known. While these inorganic membranes have advantages over membranes from organic polymers, they are also burdened with shortcomings such as brittleness, expensive and elaborate conductivity processes, and inadequate electrical conductivity. For many types of application, the electrically conductivity in particular has in recent times proved to be a property worth striving for.
Carbon presents itself as a material for membranes which do not have the drawbacks of other inorganic membranes. Therefore, membranes have already been made of carbon. J. E. Koresh and A. Sofer, in "Separation Science and Technology" (1983) 18 (8), pages 723 to 734, describe carbon membranes which are made by pyrolysis of organic compounds. Pyrolysis, even during application of relatively low temperatures of, for example, 950.degree. C., results in a carbon material which has no permeability for gases, i.e., lacks a pore system that permits mass transport from one surface of the membrane to another. Rather, such a pore system must be created by a secondary treatment. It has been found that carbon membranes with relatively large pores cannot be made by a process such as indicated in this publication, namely preoxidation and carbonization of a suitable starting material. For example, if an attempt is made to make in this way, from a polyacrylonitrile with relatively large pores going through from one surface to another, carbon membranes with relatively large through-going pores, it is found after preoxidation and carbonization that the pores have become largely closed as a result of tar formation and insufficient stability of the membrane.
The problem of inadequate stability of porous shaped polymer bodies during thermal conversion to carbon bodies is addressed in Japanese Laid-open Application 52-63428. This publication relates to the fabrication of porous filaments serving as adsorption material and having pore sizes up to a few hundred Angstroms. Pretreatment with hydroxylamine is regarded as advantageous to avoid clogged pores. Moreover, hydrazine and guanidine are mentioned as less suitable options. The hydroxylamine solutions which are described for this pretreatment may contain up to 25% of hydroxylamine. The attempt to apply the teaching of this publication to the fabrication of carbon membranes with through-going pores, particularly those with pore sizes up to a few microns, have yielded unsatisfactory results, in conformity with the information in this publication, according to which large specific surfaces were obtained only by steam treatment.
U.S. Pat. Nos. 3,977,967 and 4,341,631 disclose porous tubular bodies of carbon. These shaped items are made by a process in which a binder is carbonized. Further details on the manufacturing process are not given therein. The manufacture of carbon membranes by carbonization of a binder contained in a carrier material has the disadvantage that it is difficult selectively to control pore sizes and/or their distribution. In addition, since the membrane material is morphologically and chemically inhomogeneous, e.g., due to unremoved decomposition products of the binder, the possible uses of the membranes can be limited.